Foamed articles of polyvinyl chloride containing inorganic materials (hereinafter referred to as PVC) and processes for preparing such, have been disclosed for example, in Japanese Examined Patent Publications Nos. 26776/1977 and 7944/1978, and Japanese Unexamined Patent Publications Nos. 264232/1985 and 278543/1986.
In techniques disclosed in these publications, asbestos fibers have been incorporated into the foam as means to lessen its deforming ratin on burning, to improve its dimensional stability when used at high temperatures, and to increase its mechanical strength. Since the single fibers of asbestos are fine (diameter 0.03 to 0.01 .mu.m or so) and flexible, it is relatively easy to uniformly distribute them in cell membranes (thickness 1 to 100 .mu.m or so usually) of the foam.
In recent years, however, as the human harmfulness (carcinogenicity) of asbestos fiber has attracted considerable attention, regulation of its use also has been discussed. It is said that asbestos is harmful to health since its fibers are very fine and are easily inhaled and accumulated in pulmonary alveoli.
Therefore technology to incorporate reinforcing fibers, other than asbestos, into the foam, and such a foamed article product have been eagerly awaited. No other inorganic fiber, which is as fine and flexible as asbestos, actually exists. For example, the diameters of a single fiber of glass, rock wool, and ceramics are 10 to 13 .mu.m, 4 to 6 .mu.m, and 2 to 3 .mu.m, respectively. Thus, they are thicker and more rigid than single asbestos fibers. Such a thick and rigid fiber is apt to break through a cell membrane of the foam so that the gas therein which creates the foam may escape. For this reason, it has been through, so far, that it is technically difficult to prepare a good foamed article by incorporating a significant amount of inorganic fibers, other than asbestos, into it, and that is is further difficult to secure by such a means a good, low density foamed article which would satisfy requirements of low cost, high thermal insulation and light weight.
Meanwhile, the present inventors have developed a foamed article of chlorinated vinyl chloride resin contained inorganic material (Japanese Unexamined Patent Publication No. 264645/1988). This has advantageous characteristics in that it does not have undesirable shrinkage even in case of exposure to high temperature due to fire, and it generates little heat and little smoke compared to the amount of heat and smoke inherent in such thermoplastic resins which have high chlorine contents. Furthermore, it has a dimensional retention of not less than 70% when heated for 1 hour and an expansion ratio of not less than 5 times when heated for 1 hour at 200.degree. C.
However, the above-mentioned foamed article, based on the above-mentioned chlorinated vinyl chloride resin, has a tendency that, when it is forcedly burned (for example, in the surface test of JIS A1321), its resin parts are turned to ash, making its form retention difficult, and sometime causing generation of penetrating canals and collapse of its structure. The inventors have found that incorporating phosphoric ester is a means of alleviating such problems at least to some extent. However, they also recognized that efforts to prevent the material from turning to ash, for the purpose of surely preventing the collapse of the foam, are naturally limited under the very severe conditions of forced combustion (for example, of the model box test according to Notice No. 1231 of the Ministry of Construction (Japan)), where a flame extendibility and a flame spreading speed are used to simulate an actual fire in a closed chamber. In addition, because of the relatively high costs of chlorinated vinyl chloride resin itself and phosphoric ester, foamed articles made therefrom are also rather expensive. So there is still a need for an economically advantageous foam.
Then taking a lesson from the state of the prior art, the inventors repeated earlier studies. As a result, they found that they were able to obtain a new, low density, very good foamed article by using a combination of an inorganic substance, which contains inorganic fibers other than asbestos which are excellent with respect to their human safety and health, with a specific solvent. Surprisingly, in this new foamed article, the inorganic fiber does not break through the cell membrane of the foam, and so the blowing gas does not escape. In addition, the foamed article generates little heat, little smoke, and little noxious gas on burning, has a low heat deforming ratio on burning, is excellent in its dimensional stability and heat resistance when used at high temperatures, does not collapse due to turning to ash, and is low in cost. Thus, the present invention was attained.
Furthermore, selection of preferable conditions in the present invention made it possible to secure a foamed article which meets requirements of quasi-noncombustibility or noncombustibility specified in the Building Standard Law (Japan).